also I've seen referenced a notice from Shepperton studios when filming in Bourne, saying it would be about 300 extras and a "battle between two kingdoms ensuing before the God of Thunder himself, Thor arrives and saves the day.”

and that would again, point to "kingdoms" which I really dont think a random group of marauders hired by Thanos could possibly be considered a "kingdom". However Nornheim, Vanaheim, and even Niflheim could all certainly be considered kingdoms. And the fact that Thor has to come in with his Asgardian buddies to try to break up this fight between two kingdoms, would pretty clearly indicate that it is likely Nornheim vs Vanaheim or vs Niflheim and not Asgard vs anybody to start with (just to finish it). of course the studio notice might be misleading but I would tend to assume it's correct and take the clues from that.

also found on another site, it says someone got offered the part of a leader of a "marauder army" due to his ginger beard. now again, the word marauder being thrown around as a general term more than likely. and I would question why would he have to have a ginger beard, unless, he is meant to look like either Vanir or possibly Asgardian? Certainly wouldnt need that for a random marauder....

Spoiler!!! Click to Read!:

Well, Nornheim is part of Asgard. So if Vanaheim attacked Nornheim, I suppose that Odin and Co. would regard it as an attack on Asgard itself. So it wouldn't be the Warriors 3 et al. mediating as a more or less neutral party, but as combatants on one side.

But I do like the idea of the Vanir being part of the attacks.

I also like a lot of the elements in the great big spoiler button you have further up. It may not go that way; there's a lot of information we don't know. But it's plausible.